Dual-port testing of a cable network

ABSTRACT

A dual-port testing apparatus is provided for testing a cable network at two test points. The testing may comprise demodulation of a same data packet at the test points, decoding the data packet, performing spectral analysis of the signal, etc. Testing results may be correlated with one another, both visually and by using pre-defined test metrics comprising a weighted sum of demodulation and decoding parameters such as modulation extinction ratio and a codeword error.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention claims priority from U.S. Patent Application No.61/845,751, filed Jul. 12, 2013, entitled “Dual Cable Network Analyzer,”which is incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to cable networks, and in particular, toequipment and methods for testing cable networks.

BACKGROUND

A cable network delivers services such as digital television, Internet,and Voice-over-IP (VoIP) phone service. A cable network has acontrolling center or “head end”, which controls video and data trafficin the network by generating or distributing video and data signals. Thesignals are delivered over a tree-like network of a broadband coaxialcable termed “cable plant”. Digital television signals are broadcastfrom the headend to a trunk of the cable plant, and delivered tosubscribers' homes connected to branches of the cable plant. In goingfrom the headend to subscribers, the signals are split many times, andare attenuated in the process. Accordingly, a strong downstreambroadcast signal is required, so that the signal level at thesubscribers' premises is strong enough to be reliably detected. Upstreamsignals from the subscribers' homes carry phone and Internet traffic.The upstream signals propagate from the branches of the cable planttowards the headend of the network.

Upstream and downstream signals occupy separate frequency bands referredto as upstream and downstream frequency bands. Downstream informationchannel signals co-propagate in the downstream frequency band, andupstream signals co-propagate in the upstream frequency band. Thefrequency separation of the upstream and the downstream signals allowsbidirectional amplification of these signals, which propagate in acommon cable in opposite directions. In the United States, the upstreamspectral band typically spans from 5 MHz to 42 MHz, while the downstreamspectral band typically spans from 50 MHz to 860 MHz.

The upstream and downstream signals are prone to impairments andinterference. Oxidized connectors may act as electrical diodesdistorting the downstream signals by generating frequency harmonics,which may negatively impact both upstream and downstream signal paths.Aging equipment, such as signal boosters and amplifiers, may alsodistort the signals and add harmonics and “ringing” at unwantedfrequencies. Another source of impairments is external electricalinterference, termed “ingress noise”. Despite electrical shielding ofthe cable, outside signals may find their way into, and become guided bythe cable. Shielding punctures, especially at customers' premises,improper installation, interference from closely placed high-currentelectrical equipment, etc., all contribute to accumulation of ingressnoise. Furthermore, a cable plant may act as a receiving radio antenna.Thanks to its large size, a cable plant may pick up signals fromotherwise unlikely sources, such as aviation radars.

The impairment situation worsens as new customers are added to anexisting cable network. The cable plant is extended by adding moresplitters and connectors, amplifiers, and long runs of coaxial cable tonew locations. When a cable plant is expanded, a probability ofdownstream and upstream signal impairments increases. Accordingly,growth of extent and functionality of cable based networks must bematched by a growing effort to assure quality of existing services viaperiodic testing and maintenance of the networks.

Tracing a source of impairment is a common task in cable networkmaintenance and troubleshooting. To find an origin of noise, atechnician travels from node to node, measuring noise levels in variousbranches of the cable plant. In practice, a technician decides on theorigin of noise by taking a noise level measurement on a common leg of asignal amplifier/splitter/combiner, and comparing the measured noiselevel to noise levels on individual legs, which are connected tobranches of the cable plant. Once a “faulty” branch is identified, thetechnician consults a cable plant map, finds a location corresponding totermination of the faulty branch, travels to that location, and repeatsthe measurement.

In situations where a noise source cannot be easily identified, atechnician may be tempted to quickly disconnect a suspect branch from acable plant, to see if the noise disappears. Such practice, althoughallowing the technician to find impairments quicker, is generallydiscouraged by technician's supervisor, because it interrupts all dataand television services to many customers. With cable companies alwaystrying to improve data transfer reliability, purposefully removingservice should be avoided.

Another time-consuming problem of cable network interferencetroubleshooting is related to intermittent character of many ingressnoise sources. Noise related to bursts of defective or old cable modemsmay come at quasi-random periods of time. Furthermore, noise related toa transmission of a particular upstream channel may or may not impactthat channel, leading to puzzling situations where a detectable noisedoes not impact a particular channel, while upstream packet errorsappear on an apparently noise-free channel, due to the noise beingsomehow synchronized to the packet transmission, or occurring so rarelythat a measured frequency spectrum of a cable network signal does notshow an appreciable level of noise.

Zinevich in US Patent Application Publication 2008/0320541 discloses asystem for locating an impairment in a cable network including aplurality of “encoders” placed throughout a cable network. The functionof the encoders is to uniquely modulate the noise floor at locationswhere the encoders are installed. An “impairment detector” is placed ata headend of the cable network. The impairment detector is configured toidentify noise location(s) by analyzing noise modulation properties.While the system of Zinevich enables remote identification of noisesources in a cable network, it requires installation of many encodersthroughout the cable network, which may be costly. Furthermore,intermittent noise, and/or noise impacting only certain transmissionchannels, is not always detectable with Zinevich system.

In view of the foregoing, it may be understood that there may besignificant problems and shortcomings associated with current solutionsand technologies for testing cable networks.

SUMMARY

In accordance with an aspect of the invention, a dual-port testingapparatus is provided for simultaneous testing of a cable network at twotest points. The results of testing may be correlated with one another,both visually and by using pre-defined test metrics. Preferably, a samedata packet is captured, demodulated, and decoded at both test points,and results of demodulation and decoding compared to one another byimplementing a test metrics derived from a weighted sum of thedemodulation and decoding parameters.

In accordance with an aspect of the invention, there is provided anapparatus for testing a path of a network signal in a cable network, thepath including first and second spaced apart test points, and thenetwork signal including a first frequency channel including a firstdata packet, the apparatus comprising:

first and second RF input ports for coupling to the first and secondtest points, respectively, to obtain first and second signals,respectively, from the network signal propagating across the first andsecond test points; and

a processor coupled to the first and second RF input ports, andconfigured for: down-converting the first and second signals to selectthe first frequency channel in each one of the first and second signals;demodulating the first frequency channel in each down-converted signalto select the first packet in each down-converted signal; and obtaininga first demodulation parameter of the first packet at each test point.

In one exemplary embodiment, the processor comprises first and secondanalog to digital converters coupled to the first and second RF inputports, respectively, for digitizing the first and second signals toobtain first and second digitized signals, respectively, and a clockcoupled to the and second analog to digital converters, for synchronousclocking thereof. The processor may be configured for down-convertingthe first and second digitized signals to select the first frequencychannel in each one of the first and second signals.

In accordance with the invention, there is further provided an apparatusfor testing a path of a network signal in a cable network, the pathincluding first and second spaced apart test points, the apparatuscomprising:

first and second RF input ports for coupling to the first and secondtest points, respectively, to obtain first and second signals,respectively;

first and second analog to digital converters coupled to the first andsecond RF input ports, respectively, for synchronously digitizing thesignal coupled to the first and second RF input ports, respectively, toobtain first and second digitized signals, respectively;

a processor for performing a spectral analysis of the first and seconddigitized signals, so as to obtain first and second frequency spectra;and

a display device for displaying the first and second frequency spectrafor a visual comparison.

In accordance with another aspect of the invention, there is furtherprovided a method for testing a path of a network signal in a cablenetwork, the path including first and second spaced apart test points,and the network signal including a first frequency channel including afirst data packet, the method comprising:

(a) obtaining first and second signals from the network signalpropagating across the first and second test points;

(b) down-converting the first and second signals to select the firstfrequency channel in each one of the first and second signals;

(c) demodulating the first frequency channel in each down-convertedsignal to select the first packet in each down-converted signal, andobtaining a first demodulation parameter of the first packet at eachtest point; and

(d) displaying the first demodulation parameter corresponding to eachtest point.

In accordance with yet another aspect of the invention, there is furtherprovided a method for testing a path of a network signal in a cablenetwork, the path including first and second spaced apart test points,the method comprising:

(i) simultaneously sampling the network signal at the first and secondtest points, so as to obtain first and second digitized signals;

(ii) performing a spectral analysis of the first and second digitizedsignals, so as to obtain first and second spectra; and

(iii) displaying the first and second spectra for a visual comparison.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments will now be described in conjunction with thedrawings, in which:

FIG. 1 illustrates an exemplary view of a cable network showing how atest instrument of the invention may be connected to a pair of testpoints in the network;

FIG. 2 illustrates an exemplary view of a network process being testedusing the apparatus of FIG. 1;

FIG. 3 illustrates an exemplary block diagram of the test instrumentshown in FIG. 1;

FIG. 4 illustrates an exemplary block diagram of a demodulation andspectrum computation circuitry of the test instrument of FIG. 3;

FIG. 5 illustrates an exemplary view of a cable network showing a testinstrument of the invention connected to a pair of downstream legs of abidirectional amplifier of the network;

FIG. 6 illustrates an exemplary view of a network node being testedaccording to an alternative embodiment;

FIG. 7 illustrates an exemplary flow chart of a method for testing acable network according to the invention;

FIG. 8A shows a combined plot for frequency spectra and ingress undercarrier (IUC) at first and second test points, showing similar noisefloor levels;

FIGS. 8B and 8C show plots of a logarithmic magnitude of an error vectorper symbol as a function of symbol number at the first and second testpoints, respectively, used in computation of the IUC plots of FIG. 8A;

FIG. 9A shows a combined plot for frequency spectra and ingress undercarrier (IUC) at first and second test points, showing dissimilar noisefloor levels;

FIGS. 9B and 9C show logarithmic plots of error vector vs. symbol numberat the first and second test points, respectively, used in computationof the IUC plots of FIG. 9A;

FIG. 10 shows a combined plot for frequency spectra taken for two faultylocations, one at headend, and one at a “combined” test point;

FIG. 11 shows a combined plot for frequency spectra taken for two faultylocations, one at headend, and one at a “faulty” leg of an amplifier;and

FIG. 12 shows a combined plot for frequency spectra taken for a faultylocation and a “good” location: a “faulty” headend location and a “good”leg.

DETAILED DESCRIPTION

While the present teachings are described in conjunction with variousembodiments and examples, it is not intended that the present teachingsbe limited to such embodiments. On the contrary, the present teachingsencompass various alternatives and equivalents, as will be appreciatedby those of skill in the art.

Referring to FIG. 1, an exemplary apparatus 100 for testing a path of anetwork signal in a cable network 120 may be connected to first 101 andsecond 102 spaced apart test points of the cable network 120. The cablenetwork 120 may include a fiber node 106 including a downstreamoptoelectronic converter 106A and an upstream electro-optic converter106B, coupled via a coaxial cable 108 to a bidirectional amplifier 110,which amplifies a downstream signal 111 for distribution to first,second, and third groups of homes 116A, 116B, and 116C, respectively.The groups of homes 116A, 116B, and 116C may send first, second, andthird upstream signals 112A, 112B, and 113C, respectively, which may becombined by the bidirectional amplifier 110 into an upstream signal 112propagating towards the fiber node 106.

The test apparatus 100 may have a first 121 and second 122 radiofrequency (RF) input ports coupled to the first 101 and second 102 testpoints, respectively, of the cable network 120. If, for example, aningress noise originates in the first group of homes 116A, then theingress noise will be coupled to the first RF port 121 but not thesecond RF port 122. If an ingress noise originates in the third group ofhomes 116C, then the ingress noise will be coupled to the first RF port121 and the second RF port 122. Accordingly, the source of noise may belocated by keeping the first RF port 121 connected to the first testpoint 101, taking a combined measurement, reconnecting the second RFport 122 to third 103 and fourth 104 test points, taking anothercombined measurement, and so on.

A principle of fault location according to the invention is furtherillustrated in FIG. 2. A network process 200 may be tested by analyzingan input signal 211 of the network process 200 at a first “pre-process”test point 201 upstream of the network process 200, and at an outputsignal 212 of the network process 200 at a second “post-process” testpoint 202 downstream of the network process 200. The network process 200may include amplification, combining, spitting, and so on. If thenetwork process 200 introduces noise and/or other impairments into theoutput signal 212, noise levels and/or demodulation parameters obtainedby the test apparatus 100 by processing electrical signals at thepre-process test point 201 and post-process test point 202 may bedifferent; if not, the parameters may likely remain the same. Thus, acomparative measurement by the dual-port test instrument 100 at the twotest points 201 and 202 may indicate whether the network process 200introduces any noise or another impairment.

An exemplary construction of the test apparatus 100 will now bedescribed. Referring to FIG. 3, the test apparatus 100 may include thefirst 121 and second 122 RF input ports for coupling to the first 101and second 102 test points, respectively, to obtain first 311 and second312 signals, respectively, from a network signal 304 propagating acrossthe first 101 and second 102 test points. The network signal 304 mayinclude a first frequency channel including a first data packet, notshown. The first data packet may be a downstream or an upstream datapacket. Other frequency channels, as well as undesired ingress noise,may be simultaneously present in the network signal 304.

A processor 300 of the test apparatus 100 may be communicatively coupledto the first 121 and second 122 RF input ports. The processor 300 may beconfigured for down-converting the first 311 and second 312 signals toselect the first frequency channel in each one of the first 311 andsecond 312 signals, and for demodulating the first frequency channel ineach down-converted signal, to select the first packet in eachdown-converted signal, and to obtain a parameter of demodulation of thefirst packet captured at each test point 101 and 102.

The parameter of demodulation may include modulation error ratio (MER),noise, ingress under carrier (IUC), and any other parameter orparameters representing quality of the signal, and/or quality ofdemodulation. This parameter is denoted as “first” demodulationparameter. Herein, terms “first”, “second”, and the like, used inreference to a packet, a frequency channel, etc., are not meant todenote an order in a succession of packets or channels. Instead, suchterms are used merely for convenience, as an identifier of a packet orchannel.

In the embodiment shown, the processor 300 may include first 321 andsecond 322 analog to digital converters (ADCs) coupled to the first 121and second 122 RF input ports, respectively, for digitizing the first311 and second 312 signals to obtain first 331 and second 332 digitizedsignals, respectively, and a clock 314 coupled to the first 321 andsecond 322 ADCs, for synchronous clocking of the latter.

The test apparatus 100 may include first 341 and second 342 gain controlunits coupled between the first RF input port 121 and the first ADC 321and between the second RF input port 122 and the second ADC 322,respectively, for equalizing amplitudes of input signals 311A and 312Aof the first and ADCs, respectively, for a better comparison of thefirst 311 and second 312 signals coupled to the respective RF inputports 121 and 122.

In the exemplary embodiment shown, the processor 300 may be configuredfor down-converting the first 331 and second 332 digitized signals toselect the first frequency channel in each one of the first 331 andsecond 332 digitized signals. To that end, the processor 300 of the testapparatus 100 may include a field programmable gate array (FPGA) unit316. The clock 314 is coupled to the FPGA unit 316 for clocking the FPGAunit 316. Turning to FIG. 4, the FPGA unit 316 may include a pair ofdigital down-converters 401 and 402 for selecting the first frequencychannel in the first and second digitized signal 331 and 332,respectively, and a pair of spectrum computing units 411 and 412 forcomputing frequency spectra of the first 331 and second 332 digitizedsignals, respectively. In the exemplary embodiment shown, the spectrumcomputing units 411 and 412 may be configured to calculate frequencyspectra by fast Fourier transform (FFT).

Still referring to FIGS. 3 and 4, the processor 300 may further includea DSP processing unit 318 coupled to the FPGA unit 316. The DSPprocessing unit 318 may be configured for demodulating the firstfrequency channel in each down-converted digitized signal 331 and 332.To that end, the DSP processing unit 318 may include a demodulationcontrol module 318A. The DSP processing unit 318 may also include aspectrum computation control module 318B, for controlling the spectrumcomputing units 411 and 412 of the FPGA unit 316 (FIG. 4).

A measurement controller 320 may be communicatively coupled to the DSPprocessing unit 318, for controlling demodulation and spectracomputation by the DSP processing unit 318 and the FPGA unit 316. Themeasurement controller 320 may keep track of available channels in thecable network 120, by maintaining a channel list 320A of channels to betested.

A display processor 324 may be communicatively coupled to themeasurement controller 320. A display device 326 may be communicativelycoupled to the display processor 324. The display processor 324 may beconfigured for preparing data to be displayed, for example the frequencyspectrum and the first demodulation parameter corresponding to each testpoint 101 and 102 (FIG. 1). It should be appreciated that the displaydevice 326 may be optional because an external device, such as ahandheld tablet computer, may be used to communicate with the testapparatus 100 e.g. via a Bluetooth™ link, and to display the testresults including signal spectra, demodulation parameters, performanceindices, etc. Also, it should be appreciated that the display device 326may be configured to display the frequency spectra concurrently with thedemodulation parameters for each test point 101 and 102. Thus, frequencyspectra and demodulation parameters at the test points 101 and 102 maybe directly compared to each other. Since the demodulation parametersrelate to a same data packet, the test apparatus 100 may allow thecomparison of the first 311 and second 312 signals received at the first121 and second 122 RF input ports, respectively, to be performed onpacket-by-packet basis. This configuration may be advantageously used totroubleshoot intermittent errors. For instance, the measurement controlprocessor 320 of the testing apparatus 100 may be configured to onlydisplay results related to a packet having significant demodulationerrors exceeding a pre-defined threshold. Thus, the results displayed inthe display 326 may allow a user to perform comparison of demodulationerrors at the first 101 and second 102 test points. If, for instance,the demodulation errors are similar, then the problem resides outside ofa portion of a cable network transmission path between the first 101 andsecond 102 test points.

The test apparatus 100 may be used to test not only an upstream, butalso a downstream signal path. Turning to FIG. 5, the test apparatus 100may be connected to the test points 104 and 102 associated with thefirst group of homes 116A and the third group of homes 116C,respectively. The downstream signal 111 may be split into portions 111A,111B, and 111C destined for the first 116A, the second 116B, and thethird 116C groups of homes, respectively. The first 116A and third 116Cportions may be sampled by the apparatus 100. Referring to FIG. 6, anoise signal 611 may be split into portions 611A, 611B, and 611C, thatis, the noise signal 611 may propagate along the paths of the downstreamsignal 111. In this scenario, the detected demodulation and packeterrors and the signal spectra obtained by the test apparatus 100 wouldbe similar. If they are different, then a source of impairment may belikely located downstream of a leg corresponding to the largestdemodulation errors, or the largest noise.

In one embodiment of a dual testing apparatus of the invention, thedemodulation circuitry may be omitted, and the comparison may be basedsolely on synchronously captured spectra at a pair of spaced apart testpoints. Referring back to FIG. 3, the first 121 and second 122 RF inputports of such an apparatus may be coupled to first 321 and second 322ADCs coupled to the first 121 and second 122 RF input ports,respectively, for synchronously digitizing signals coupled to the first121 and second 122 RF input ports, respectively, to obtain the first 331and second 332 digitized signals, respectively. In this particularembodiment, the processor 300 may be configured only for performing aspectral analysis of the first 331 and second 332 digitized signals,preferably via FFT, so as to obtain the first and second frequencyspectra. The spectra may be displayed on the display device 326 for avisual comparison.

Referring to FIG. 7 with further reference to FIGS. 1 and 3, a method700 (FIG. 7) for testing a path of the network signal e.g. 304 (FIG. 3)in the network 120 using the test apparatus 100 (FIG. 1) may include astep 702 (FIG. 7) of obtaining the first 311 and second 312 signals fromthe network signal 304 propagating e.g. between the first 101 and second102 test points (FIG. 1). In a step 704, the first and second signalsmay be down-converted, e.g. by the FPGA 316, to select the firstfrequency channel in each one of the first 121 and second 122 signals.In a step 706, the first frequency channel in each down-converted signalmay be demodulated to select the first packet in each down-convertedsignal. Parameters of demodulation, such as MER, IUC, etc. may becollected in this step. The first demodulation parameter of the firstpacket may be collected at each test point 121 and 122. Then, in anoptional step 708, the first packet may be decoded, and a first decodingparameter, such as a codeword or another decoding error, is obtained foreach one of the first 101 and second 102 test points. Severaldemodulation and several decoding parameters of the first packet may becollected. In an optional step 710, a performance index (PI) may becomputed based on a weighted sum of all collected demodulation anddecoding parameters. Finally, in a step 712, the first demodulationand/or the first decoding parameter and/or PI corresponding to each testpoint may be displayed on the display 326, or on an external display(not shown).

The method 700 may be implemented in the processor 300 of the testapparatus 100. In particular, the processor 300 may be configured forcomputing a MER for each demodulated symbol of each first packet. Theprocessor 300 may also be configured to obtain other demodulation and/orone or more decoding parameters of the first packet at each test point.The display processor 324 and the display device 326 may be configuredto display the obtained demodulation and/or decoding parameters of thefirst packet for each test point, as well as graphs of the MER as afunction of a demodulated symbol number, for each test point. Examplesof such graphs are provided below.

In the method 700, the first 101 and second 102 test points may bedisposed on the common and downstream legs of the amplifier 110. Such aconfiguration may allow one to trace propagation of upstream ordownstream signal through the bidirectional amplifier 110.Alternatively, the common leg may be excluded, that is, the test pointsmay be disposed as shown in FIG. 6. The latter configuration may be usedfor testing downstream signal paths.

Preferably, the signal obtaining step 702 may include simultaneouslysampling the network signal 304 at the first 101 and second 102 testpoints, so as to obtain the first 331 and second 332 digitized signals.The simultaneous or clock-synchronized sampling may ensure that a samepacket may be captured in nearly identical conditions of digital signalprocessing. In this embodiment, the down-converting step 704 may beperformed digitally, that is, using the first 331 and second 332digitized signals to select the first frequency channel in each one ofthe first 331 and second 332 digitized signals.

A step 714 of performing spectral analysis of the first 311 and second312 signals may be performed independently from demodulation/decoding/PIcomputation steps 704 to 710. In one embodiment, the spectral analysisstep 714 may include a step 722, in which the signal 304 is sampledsimultaneously at the first 101 and second 102 test points, so as toobtain the first 331 and second 332 digitized signals. In a next step724, the spectral analysis may be performed of the first 331 and second332 digitized signals, so as to obtain first and second frequencyspectra. Finally, in the displaying step 712, the first and secondspectra may be displayed together, e.g. side by side, for a visualcomparison. If the demodulation and decoding are performed on one packetor several consecutive packets, the corresponding demodulation/decodingparameters and/or PI for each test point and each packet may bedisplayed along with the frequency spectra.

To reduce a possibility of false differences between the first 121 andsecond 122 test points due to intermittent noise, the sampling step 724may be achieved in a synchronous fashion, e.g. the ADCs 321 and 322 aresynchronously clocked by the single clock 314 (FIG. 3). Also in thefirst step 722, the first 101 and second 102 test points may be disposedon legs of the bidirectional amplifier 110 including a combined leg.Alternatively, the combined leg may be excluded, that is, the testpoints may be disposed as shown in FIG. 6. Different test pointcombinations may be tried to determine which leg carries noisy orotherwise compromised signal. The analysis step 724 may include FFT. Inthe displaying step 712, the FFT spectra may be displayed in anoverlapped fashion or side-by-side for ease of comparison. Adifferential spectrum may also be computed to highlight the differencesbetween the two test points.

Network testing results using the apparatus 100 of FIGS. 1, 3, and 6,and the method 700 of FIG. 7 are presented in FIGS. 8A to 8C; 9A to 9C;and FIGS. 10 to 12. In FIGS. 8A, 9A, the spectra span from 4 MHz to 85MHz; and in FIGS. 10 to 12, the spectra span from 4 MHz to 42 MHz. Theamplitude scales on the left and on the right in dBmV correspond tologarithms of signal amplitudes at the first 121 and second 122 RF inputports of the apparatus 100 shown in FIGS. 1 and 3. The scales differencecompensates for a difference in measured signal magnitudes at the first121 and second 122 RF input ports.

The testing was performed using a test bed cable network systemincluding a headend, the fiber node 106, the coaxial cable span 108, andthe bidirectional amplifier 110 (FIG. 1). All the equipment wascollocated for testing purposes. In a first test, the first testlocation was at the headend, and the second test location correspondedto the second test point 102. Referring specifically to FIG. 8A,upstream band frequency spectra 801 and 802 of the first and secondsignals 311 and 312 (FIG. 3) are shown for a case when the two locationsshow similar performance. In FIG. 8A, the spectra 801 and 802 are almostidentical.

Referring to FIGS. 8B and 8C, an error vector per symbol is plotted indB units as a function of symbol number in the first packet of the firstsignal 311 (FIG. 8B) and the second signal 312 (FIG. 8C). The similarityof graphs of FIGS. 8B and 8C indicates that the signal quality of thefirst and second signals 311 and 312 is similar. Referring back to FIG.8A, ingress under carrier (IUC) in the first frequency channel denotedat 810 is plotted for the first 311 and second 312 signals at 811 and812, respectively. The IUC plots 811 and 812 in the first frequencychannel 810 are obtained by taking Fourier transforms of thecorresponding error vector data shown in FIGS. 8B and 8C, respectively.The conclusion of similarity of the signal quality of the first andsecond signals 311 and 312 is corroborated by the similarity of IUCplots 811 and 812, indicating that a potential impairment source is notlocated in a path between the second test point 102 and the headend.

Turning to FIG. 9A, upstream band frequency spectra 901 and 902 of thefirst and second signals 311 and 312 (FIG. 3) illustrate a case wherethe signal quality at the two simultaneously measured locations, one atthe headend, and one at the third test point 103 (FIG. 1), is different.Specifically, the first spectrum 901, measured at the headend, shows amuch higher noise floor between 4 MHz and 12 MHz and around 20 MHz thanthe second spectrum 902, measured at the third test point 103. First 911and second 912 IUC plots in the first frequency channel 810 for thefirst 311 and second 312 signals, respectively, also show a remarkabledifference. The first IUC plot 911 show much higher levels of noise thanthe second IUC plot 912 only peaking at a single frequency. Thisindicates that the impairment was not present at the test point 103,originating somewhere upstream of the third test point 103 towards theheadend. In this case, the impairment was actually caused by theupstream signal level originating at the test point 103 being too high,and causing oversaturation of laser at the upstream electro-opticconverter 106B (FIG. 1). Referring to FIGS. 9B and 9C, the MER is alsoremarkably different for the two test points.

Turning to FIG. 10 with further reference to FIG. 1, a headend upstreamfrequency spectrum 1001 (FIG. 10) is plotted together with an upstreamfrequency spectrum 1002 measured at the first test point 101,corresponding to the combined leg of the amplifier 110 (FIG. 1). Thespectra 1001 and 1002 are very similar. Demodulation and decoding of asame upstream packet was performed for both signals according to themethod 700 of FIG. 7, and a performance index (PI) was calculated basedon a weighted sum of the corresponding demodulation and decodingparameters. The PI varies from 0 (no signal) to 100 (a perfect signal).For both spectra 1001 and 1002, the PI was equal to 65, indicating asomewhat compromised performance.

Referring to FIG. 11 with further reference to FIG. 1, a headendupstream frequency spectrum 1101 (FIG. 11) is plotted together with anupstream frequency spectrum 1102 measured at the second test point 102,corresponding to an “impaired” leg of the amplifier 110 (FIG. 1). Thespectra 1101 and 1102 are again very similar, indicating that theperformance is still compromised on that leg. The PI calculations forboth test points resulted in a same value of 66, confirming thisconclusion.

Turning now to FIG. 12 with further reference to FIG. 1, a headendupstream frequency spectrum 1201 (FIG. 12) is plotted together with anupstream frequency spectrum 1202 measured at the third test point 103,corresponding to a “good” leg of the amplifier 110 (FIG. 1). The spectra1201 and 1202 are markedly different: while the first spectrum 1201,corresponding to the headend test point, has a relatively high noisefloor, the second spectrum is much less noisy. The PI calculationsconfirmed this conclusion, having yielded the values of 68 and 98 forthe headend test point and the third test point 103, respectively. Thegraphs 801, 802 of FIG. 8A; 901, 902 of FIG. 9A; 1001, 1002 of FIG. 10;1101, 1102 of FIGS. 11; and 1201, 1202 of FIG. 12, as well as the MERplots of FIGS. 8B, 8C and 9B, 9C, were displayed on the display device326 of the test apparatus 100 along with the corresponding PI values.Based on these graphical data, a technician may determine a noiseorigin.

The hardware used to implement the various illustrative logics, logicalblocks, modules, and circuits described in connection with the aspectsdisclosed herein may be implemented or performed with a general purposeprocessor, a digital signal processor (DSP), an application specificintegrated circuit (ASIC), a field programmable gate array (FPGA) orother programmable logic device, discrete gate or transistor logic,discrete hardware components, or any combination thereof designed toperform the functions described herein. A general-purpose processor maybe a microprocessor, but, in the alternative, the processor may be anyconventional processor, controller, microcontroller, or state machine. Aprocessor may also be implemented as a combination of computing devices,e.g., a combination of a DSP and a microprocessor, a plurality ofmicroprocessors, one or more microprocessors in conjunction with a DSPcore, or any other such configuration. Alternatively, some steps ormethods may be performed by circuitry that is specific to a givenfunction.

The foregoing description of one or more embodiments of the inventionhas been presented for the purposes of illustration and description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed. Many modifications and variations are possiblein light of the above teaching. It is intended that the scope of theinvention be limited not by this detailed description, but rather by theclaims appended hereto.

What is claimed is:
 1. An apparatus for testing a path of a networksignal in a cable network, the path comprising first and second spacedapart test points, the network signal comprising a first frequencychannel comprising a first data packet, the apparatus comprising: afirst RF input port for coupling to the first test point to obtain afirst signal from the network signal propagating across the first testpoint, and a second RF input port for coupling to the second test pointto obtain a second signal from the network signal propagating across thesecond test point; and a processor communicatively coupled to the firstand second RF input ports, and configured to: down-convert the first andsecond signals to select the first frequency channel in each one of thefirst and second signals; demodulate the first frequency channel in eachdown-converted signal to select the first packet in each down-convertedsignal; and obtain a first demodulation parameter of the first packet ateach test point.
 2. The apparatus of claim 1, wherein the processorcomprises a first analog to digital converter coupled to the first RFinput port for digitizing the first signal to obtain a first digitizedsignal, and a second analog to digital converter coupled to the secondRF input port for digitizing the second signal to obtain a seconddigitized signal, and a clock coupled to the first and second analog todigital converters, for synchronous clocking thereof; wherein theprocessor is configured for down-converting the first and seconddigitized signals to select the first frequency channel in each one ofthe first and second digitized signals.
 3. The apparatus of claim 2,further comprising a first gain control unit coupled between the firstRF input port and the first analog to digital converter, and a secondgain control unit coupled between the second RF input port and thesecond analog to digital converter, for equalizing amplitudes of inputsignals of the first and second analog to digital converters.
 4. Theapparatus of claim 3, further comprising a display devicecommunicatively coupled to the processor, for displaying the firstdemodulation parameter corresponding to each test point.
 5. Theapparatus of claim 4, wherein the processor comprises a digitaldown-converter for selecting the first frequency channel in eachdigitized signal, and a spectrum computing unit for computing frequencyspectra of each digitized signal; wherein the display device isconfigured to display the frequency spectra concurrently with thedemodulation parameters for each test point.
 6. The apparatus of claim5, wherein the processor comprises: an FPGA unit having implementedtherein the digital down-converter and the spectrum computing unit,wherein the clock is coupled to the FPGA unit for clocking the FPGA; aDSP processing unit coupled to the FPGA unit, wherein the DSP processingunit is configured to demodulate the first frequency channel in eachdown-converted digitized signal; a measurement controller coupled to theDSP processing unit, for controlling demodulation and spectracomputation by the DSP processing unit and the FPGA unit; and a displayprocessor coupled to the measurement controller and the display device,for preparing data to be displayed by the display device, wherein thedata comprises the frequency spectrum corresponding to each test point,and the first demodulation parameter corresponding to each test point.7. The apparatus of claim 1, wherein the processor is configured tocompute a modulation error ratio for each demodulated symbol of eachfirst packet, and wherein the display is configured to display graphs ofthe modulation error ratio as a function of a demodulated symbol number,for each test point.
 8. The apparatus of claim 1, wherein the processoris further configured to decode the first packet received at each testpoint, to obtain a first decoding parameter of the first packet at eachtest point; and wherein the display is configured to display the firstdecoding parameter of the first packet for each test point.
 9. Theapparatus of claim 8, wherein the processor is further configured toobtain a plurality of demodulation parameters at each test point,comprising the first demodulation parameter, and a plurality of decodingparameters at each test point, comprising the first decoding parameter,wherein the processor is further configured to compute, for each testpoint, a performance index from a weighted sum of the correspondingdemodulation and decoding parameters; and wherein the display isconfigured to display the performance index of the first packet for eachtest point.
 10. An apparatus for testing a path of a network signal in acable network, the path comprising first and second spaced apart testpoints, the apparatus comprising: a first RF input port for coupling tothe first test point to obtain a first signal, and a second RF inputport for coupling to the second test point to obtain a second signal;first and second analog to digital converters coupled to the first andsecond RF input ports, respectively, for synchronously digitizing thesignal coupled to the first and second RF input ports, respectively, toobtain first and second digitized signals, respectively; a processor forperforming a spectral analysis of the first and second digitizedsignals, so as to obtain first and second frequency spectra; and adisplay device for displaying the first and second frequency spectra fora visual comparison.
 11. A method for testing a path of a network signalin a cable network, the path comprising first and second spaced aparttest points, the network signal comprising a first frequency channelcomprising a first data packet, the method comprising: (a) obtainingfirst and second signals from the network signal propagating across thefirst and second test points; (b) down-converting the first and secondsignals to select the first frequency channel in each one of the firstand second signals; (c) demodulating the first frequency channel in eachdown-converted signal to select the first packet in each down-convertedsignal, and obtaining a first demodulation parameter of the first packetat each test point; and (d) displaying the first demodulation parametercorresponding to each test point.
 12. The method of claim 11, whereinstep (a) comprises simultaneously sampling the network signal at thefirst and second test points, so as to obtain first and second digitizedsignals, wherein step (b) comprises down-converting the first and seconddigitized signals to select the first frequency channel in each one ofthe first and second digitized signals.
 13. The method of claim 11,wherein step (c) comprises computing a modulation error ratio for eachdemodulated symbol of each first packet, and wherein step (d) comprisesdisplaying graphs of the modulation error ratio as a function of ademodulated symbol number, for each test point.
 14. The method if claim11, wherein step (c) comprises computing a modulation error ratio foreach demodulated symbol of each first packet, and performing FFT offunctions of the modulation error ratio versus demodulated symbolnumber, so as to obtain frequency spectra of ingress under carrier ineach first packet, wherein step (d) comprises displaying the frequencyspectra of ingress under carrier for each test point.
 15. The method ofclaim 11, further comprising (c1) upon completion of step (c), decodingthe first packets at each of the first and second test points, andobtaining a first decoding parameter of the first packet at each testpoint; wherein step (d) comprises displaying the first decodingparameter of the first packet for each test point.
 16. The method ofclaim 15, wherein step (c) comprises obtaining a plurality ofdemodulation parameters at each test point, comprising the firstdemodulation parameter, and step (c1) comprises obtaining a plurality ofdecoding parameters at each test point, comprising the first decodingparameter, the method further comprising (c2) upon completion of step(c1), computing, for each test point, a performance index from aweighted sum of the corresponding demodulation and decoding parameters;wherein step (d) comprises displaying the performance index of the firstpacket for each test point.
 17. The method of claim 11, wherein in step(a), the first and second test points are disposed on first and secondlegs of an amplifier, comprising a combined leg thereof.
 18. A methodfor testing a path of a network signal in a cable network, the pathcomprising first and second spaced apart test points, the methodcomprising: (i) simultaneously sampling the network signal at the firstand second test points, so as to obtain first and second digitizedsignals; (ii) performing a spectral analysis of the first and seconddigitized signals, so as to obtain first and second spectra; and (iii)displaying the first and second spectra for a visual comparison.
 19. Themethod of claim 18, wherein step (ii) comprises performing a fastFourier transform of the first and second digitized signals.
 20. Themethod of claim 18, wherein in step (i), the first and second testpoints are disposed on first and second legs of an amplifier, excludinga combined leg thereof.